How You Can Keep Your CP4.2 High-pressure Fuel Pump Off the Scrap Pile

By now you’ve heard of all the problems surrounding the Bosch CP4.2, the high-pressure fuel pump that’s made its way onto most common-rail engines in the pickup truck segment. You know the story well: the pump self-destructs, sends metal fragments through the lines and rails, into the injectors, and then out the return circuit. The failure is rarely noticed before it’s too late, and by then you’re usually out a pump, lines, rails, injectors, a tank cleaning, 30 hours’ labor, and anywhere between $6,000 to $10,000. As expected, this potential catastrophe has many late-model truck owners a little uneasy about their $60,000 to $90,000 investments.

With its ultra-tight tolerances, the CP4.2 does not tolerate anything other than diesel fuel very well. But which unwanted contaminant is most common? Air. That’s right, aeration is the number one killer of these pumps. Whether it be from lack of maintenance, running the truck out of fuel, improper fuel filter installation, or not allowing the fuel system to properly prime after a filter change, nine times out of 10 this is what takes out the CP4.2. We recently stopped by RCD Performance’s state-of-the-art fuel injection shop for the full rundown on how the CP4.2 operates, what its key failure points are, and how the average truck owner can prevent it from failing.

CP4.2 Fuel Pump for the 6.7L Power Stroke and LML Duramax
A radial piston pump, the Bosch CP4.2 consists of two high-pressure cylinders (hence the “2” in CP4.2), each with its own piston assembly, spring, plunger, and head. A two-lobed camshaft drives the pistons (sometimes referred to as buckets), with each piston being actuated twice per crankshaft revolution for a total of four compression strokes. The CP4.2 must be timed with both the engine’s crankshaft and camshaft to coincide perfectly with the injectors opening, and in both the 6.7L Power Stroke and LML Duramax the pump is driven by the engine’s camshaft.
Bosch CP4.2 and the CP3 fuel pump
While it achieves the same goal (highly pressurized fuel), the CP4.2 is noticeably different from its predecessor, the CP3. The CP4.2 (left) is a twin-piston pump while the CP3 (right) utilizes three, but thanks to its dual-lobe cam the pistons in the CP4.2 perform twice the work. Although the CP4.2 moves less fuel volume than the CP3 (roughly 20-percent less according to the output of LML Duramax CP4.2’s), it is substantially more efficient and produces higher peak pressure (30,000 psi vs. 26,000 psi).
external high-pressure fuel circuits on the Bosch CP4.2
Also unlike the CP3, all high-pressure fuel circuits are external on the CP4.2. So instead of requiring an extensive (and expensive), forged-steel and heat-treated housing, all the CP4.2’s parts are installed within (or bolted to) an aluminum case.
Bosch CP4.2 piston and roller tappet assemblies, piston springs, plungers, heads, and volume control valve
Broken down, the piston and roller tappet assemblies, piston springs, plungers, heads, and volume control valve (VCV) are all visible here. When compared with a CP3, there are considerably less moving parts, which makes the CP4.2 both more cost-effective to produce and less complex in overall design.
CP4.2 fuel pump piston assembly
The design of the CP4.2 contains the entire high-pressure area within the cylinders. A piston assembly consisting of the piston, roller tappet, plunger, and spring sits in each cylinder and is topped off with a steel head. Integrated high-and-low pressure valves are present in the heads as well.
CP4.2 Fuel Pump internal integrated ports and valves, low-pressure fuel inlet, high-pressure check valve, and high-pressure outlet
Among the integrated ports and valves found in the heads are the low-pressure fuel inlet, the high-pressure check valve, and the high-pressure outlet that allows fuel to enter the rail. Like the CP3, fuel passages within the CP4.2 are fairly sizeable. It’s unlikely they could even be a restriction in modified versions of the CP4.2.
CP4.2 fuel pump high-pressure check valve
The high-pressure check valve is located at the top of each head. The job of this one-way valve is to ensure fuel cannot return to the low-pressure circuit once it’s entered the high-pressure area. An outlet check valve is also utilized. It closes when low-pressure fuel is being introduced into the pump due to pressure differentiation, but opens during the piston’s compression stroke.
CP4.2 fuel pump plunger and high-tension spring
A plunger operates above each piston, surrounded by a high-tension spring, and is akin to a connecting rod in this instance. The spring is responsible for returning the piston to the rest position following each compression stroke.
CP4.2 cam bearing housing
Access to the cam is gained by removing the cam bearing housing, which is fastened to the front of the CP4.2 housing. The cam bearing housing employs a sleeve bearing to support the cam and is secured via four Torx head bolts.
CP4.2 internal supply ports
With the cam removed, the internal supply ports are visible. The port on the right routes fuel up to the plunger and relief valve (accessible on the back side of the pump), while the port on the left is for supply feeding into the pump.
CP4.2 fuel pump camshaft
Surprisingly similar in size and shape with what is found inside the CP3, the CP4.2’s camshaft features two actuating lobes. The lobes are offset 180 degrees from each other and as was previously mentioned each piston is actuated two times per revolution of the engine’s crankshaft. In addition to the roller tappet at the bottom of the piston (shown here behind the cam), the cam lobes bear the brunt of the damage when a CP4.2 fails.
Bosch CP4.2 roller tappet
As the cam rotates, its lobes push the pistons up in their respective bores. The bottom of each piston is fitted with a roller tappet (shown), which rides on the cam lobe. The roller tappet is pressed into a polished follower.
Bosch CP4.2 cam lobes
This is how the roller tappet on the bottom of the piston rides on the cam. Much different from the CP3, where the cam lobes push directly on the plungers, the point of interaction between the cam lobes and the roller tappets relies heavily on a layer of fuel being present between the two. If lubrication is lost, the roller will eventually seize.
Bosch CP4.2 fuel pump piston assembly
Unfortunately, there is no provision in place in the CP4.2 to keep the piston from rotating within its bore. After tracing the failures it’s seen in dozens of LML Duramax applications back to either the system’s lack of an electric low-pressure fuel supply pump, improper installation of the fuel filters, or the end-user running the truck completely out of fuel, the folks at RCD Performance believe aerated fuel is the primary cause behind CP4.2 failures. With highly aerated fuel inside the piston, the piston assembly is allowed to float, which often results in the piston rotating.
Bosch CP4.2 with damaged roller tappet and cam
As you can imagine, once the piston has rotated within its bore (often 90 degrees) and the roller tappet is perpendicular to the cam lobe, the relationship between the two drastically changes. And with fluctuations between highly pressurized fuel and highly aerated fuel bearing down on it from above, the roller tappet effectively becomes a pile driver. Excessive wear of both the roller tappet and cam occurs rapidly at this point, but most drivers don’t notice a problem until the truck stutters and shuts off.
diagram of a CP4.1 internal fuel pump
As the roller tappet is usually the first component to break down during failure, this diagram (of a CP4.1) helps explain why the rest of the fuel system becomes contaminated with metal debris. As you can see, shrapnel-laced fuel from the destruction of the roller tappet not only circulates around the cam, but also makes its way into the high-pressure chamber and out the high-pressure outlet (as well as the volume control valve). Not only do the injectors see metal shards, but so does the return side of the fuel system, which carries it all the way back to the tank.
CP4.2 fuel pump volume control valve
One of the first places you’ll discover CP4.2 failure is at the volume control valve (VCV). Known as the FCA, MPROP, or fuel pressure regulator in CP3 applications, the VCV precisely meters the flow rate of fuel going into the cylinders. As the metal-on-metal contact between the roller tappet and cam begin to produce steel fragments, a plugged 80-micron screen on the VCV will tell you everything you need to know.
The primary difference between the CP4.2 used on the 6.7L Power Stroke vs. the unit found on the LML Duramax is Ford’s utilization of an electric low-pressure pump supplying diesel to it. GM’s version does incorporate a gear pump on the back side to get positive pressure heading toward the CP4.2 (shown), but as the un-salvageable LML cores pile up at RCD Performance, it’s clear that the help provided by the mechanical pump isn’t enough.
This is believed to be the dominant reason CP4.2 failure is less common on the 6.7L Power Stroke than it is on the LML Duramax: the electric lift pump that’s part of the all-in-one diesel fuel conditioning module (primary filter, drain valve, and pump). Not just any low-pressure fuel supply pump, Ford makes sure 55 to 65 psi is being sent to the CP4.2 at all times. Any time fuel supply pressure drops below 50-psi for a sustained interval, a low fuel pressure code will be thrown.
Ford 6.7L Power Stroke engine
Although it’s not as common for a pump failure to occur on the 6.7L Power Stroke, if the CP4.2 does disintegrate, you’re replacing the fuel cooler and possibly even the low-pressure fuel pump in addition to the CP4.2, lines, rails, injectors, and necessary sensors. To keep the CP4.2 in a 6.7L Power Stroke happy, maintain a strict fuel filter change regimen (performed at or before Ford’s recommended interval), make sure you change both the primary and secondary filters, ensure you install them properly, and always fill your tank with quality fuel from a safe, reputable source.
GMC LML Duramax 6.6L Turbo Diesel
Many of us know the struggle that can sometimes be priming the Duramax’s fuel system following a fuel filter change, but this is often where a lot of the CP4.2’s troubles begin. According to RCD Performance, many owners start the engine before proper priming has been performed by hand, and then rev the engine up in an effort to push the air through the system before the engine dies. As far as the CP4.2 is concerned, this is the worst thing you can do. The best course of action is to prime the hand-pump until it’s tight, start the engine, continue priming the filter until all air is bled out, and allow the engine to idle for five minutes prior to any type of acceleration. In BMW applications that employ a CP4-based pump, it’s recommended that a program on OE scan tools be used to purge the fuel system by way of idling the engine while the VCV is cycled. The fact that this elaborate process is standard operating procedure for such a seemingly simple maintenance item leads us to believe BMW knows how damaging air can be for the CP4.
Motorcraft Power Stroke Fuel Filter FD-4615
Who knew something as innocent as improperly installing a fuel filter could lead to such catastrophic fuel system failure? Due to its ability to allow air to infiltrate the high-pressure system, the guys at RCD Performance say it’s possible. Aerated fuel creates a host of issues inside fuel systems, the worst of which is lack of lubrication and inconsistent pressure. While changing out filters seems idiot-proof, if you’re doing it on your truck for the first time take a peek at the owner’s manual, just to be sure. In 6.7L Power Stroke applications, turn the key to the on position and listen for the low-pressure fuel pump. If you pay attention, you can hear the air-fuel mixture burp in the tank after two or three cycles. Then let the truck idle for five minutes to ensure all air is gone before accelerating.
CP4.2 fuel pump cylinder bores
We’ve been told several possible mechanical remedies are in the works to keep the pistons from rotating in the CP4.2’s cylinder bores. The version we think makes the most sense (the simplest, most affordable route) would be the addition of a keyway. The aeration issue on the other hand can never be completely ruled out, but it’s a good idea for LML Duramax owners to add an aftermarket electric lift pump (cheaper path) or switch to a CP3 (more expensive path), and it behooves owners of all CP4.2-equipped engines to practice regular and proper fuel filter maintenance schedules.

Bosch CP4.2:

A Compact, Efficient Platform That’s Here to Stay

While most of us know the Bosch CP4.2 because it’s what came on our LML Duramax or 6.7L Power Stroke, this high-pressure fuel pump has been around a while. With 39,000 psi (2,700 bar) capability, the CP4 can meet even the most stringent diesel emission standards, and it’s been used in conjunction with both piezo and solenoid style injectors in dozens of passenger car makes and models all over the world. To date, more than 40 million CP4’s have been produced, with its modular design allowing Bosch to grow (CP4.2) or scale down the pump (CP4.1) based on the OE’s fuel system requirements and packaging needs.

The Bosch CP4.2 debuted in the North American truck segment back in 2011 on both the 6.7L Power Stroke and the LML Duramax, although a similar version (CP4.1) had already been brought to other markets such as Volkswagen, BMW, and more than 20 other brands, globally. Fast-forward eight years and it’s in use in most diesel engines in the truck world. Though GM moved on to the Denso HP4 pump for its L5P Duramax in ’17, you can still find the CP4.2 aboard the aforementioned 6.7L Power Stroke, the 3.0L VM Motori EcoDiesel in Ram 1500’s, the 5.0L Cummins in the Nissan Titan XD, and now even the new 6.7L Cummins uses one. One thing’s for sure: this pump isn’t going anywhere any time soon.


RCD Performance


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